The present invention relates to an interface circuit and, more particularly, to an interface circuit for fire alarm, security or other single or multiple sensors connected in parallel.
In most sensor systems, one or more sensors are connected to a common monitor/control device via an interface circuit. The interface circuit between the electrical loop formed by the sensors and the monitor/control device must sense when the loop current exceeds a predetermined value and produce an appropriate output signal at that time, the loop current being determined by the number and type of sensor in the loop.
Conventional interface circuits perform a current to voltage conversion and use voltage comparison techniques to monitor the voltage developed across a known resistance through which the loop current is flowing. This requires either a high power resistor or a precision small resistor to accomplish the conversion and changes the voltage applied to the loop when the loop load changes, which in turn changes the sensitivities trip point of the monitoring devices or sensors across the loop. The voltage which is developed across the sensing resistor is amplified and applied to a latching device within the interface circuit, the latching device typically being a relay. Temperature dependent gains of transistors, amplifiers and comparators used in these circuits greatly affect the accuracy with which the loop current levels can be determined.
This present invention monitors an accurate fraction of the loop current by using multiple diode-connected transistors directly in the loop path which reflect a fraction of the loop current into a current mirror and eliminates the need for high power or precision small resistors in the current to voltage conversion circuit. Power consumption within the interface circuit remains low, eliminating the changing resistance problem associated with power dissipation in the sensing resistor and close proximity parts, thereby improving accuracy. The use of a current mirror passing only a part of the loop current reflected by the diode-connected transistors rather than sensing resistors provides high temperature stability and causes a constant voltage to be maintained across the sensor loop, thereby eliminating the problem of monitoring devices changing sensitivities as one or more of the sensors in the loop actuate. Current limiting reduces the complexity of the mating control equipment and provides protection for the devices in the loop. A solid state latching function within the interface circuit increases reliability and reduces size and cost.